Tom Thornton calls it the “wow” factor. The third-party applications built for Apple’s iPhone and iPod Touch have plenty of it. “It’s ‘Wow,’ I can’t believe a phone can do this now,” says Thornton, a senior research science at usability specialist Perceptive Sciences in Austin, Texas. 

But there’s a dark side to wow moments, too. Thornton had one of those when he was fooling around with MyWeather. He discovered the weather application’s best feature–a nifty, full-screen depiction of the weather–entirely by accident when he inadvertently tilted his phone on one side. That “wow,” Thornton says wryly, translated into something like, why couldn’t the developer leave users a hint about the feature. 

“There is absolutely nothing telling you that,” Thornton says. “A little extra messaging, a little extra guidance, would help.” 

The hundreds of new applications for Apple’s (nasdaq: AAPL - news -people ) iPhone and iPod Touch digital media player have created a sensation among the device owners. Within a month of the July launch of the App Store, Apple had sold $30 million worth of novel applications for its devices–a particularly impressive number when you remember much of the software is free. 

But as fresh hoards of developers start to create applications, they could trample on what has been the sterling virtue of Apple’s products–intuitive and consistent usability. And if users fill their devices with applications that lack Apple’s knack for crafting easy-to-use software, the overall usability of the iPhone could diminish, Thornton suggests.

Of course, Thornton knows that trying out wild new applications are part of the fun, too. The iPhone is packed with an unusual combination of capabilities, and the phone’s latest software gives developers access to practically all of them. That’s likely to lead to a spate of applications that will wow even hardcore technophiles. For example, one application, Scribular, takes advantage of the phone’s link to the global satellite navigation system to allow users to leave virtual graffiti that is linked to a physical location but only lives in cyberspace that other users can see. More>>